Electronic display devices, such as monitors used with computers and screens built in to telephones and portable information devices, are usually designed to have a viewing angle as wide as possible so that they can be read from any viewing position. However, there are some situations where a display which is visible from only a narrow range of angles is useful. For example, one might wish to read a private document using a portable computer while on a crowded train.
A number of devices are known which restrict the range of angles or positions from which a display can be viewed.
U.S. Pat. No. 6,552,850 discloses a technique for the display of private information on a cash-dispensing machine. Light emitted by the machine display has a fixed polarisation state and the machine and its user are surrounded by a large screen of sheet polariser which absorbs light of that polarisation state but transmits the orthogonal state. Passers by can see the user and the machine but cannot see information displayed on the screen.
Another known technique for controlling the direction of light is a ‘louvred’ film. The film consists of alternating transparent and opaque layers in an arrangement similar to a Venetian blind. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction nearly parallel to the layers but absorbs light travelling at large angles to the plane of the layers. These layers may be perpendicular to the surface of the film or at some other angle.
As an example, U.S. Pat. No. RE27,617 describes a polymer film which contains absorbing ‘microlouvre’ structures. The polar angle, q, (shown in FIG. 1, which illustrates the co-ordinate system used herein) at which privacy occurs at is a function of the louvre's height to width ratio. (In FIG. 1, the in-plane (azimuth) and out-of-plane (polar) angles are indicated as f and q, respectively; the LCD panel is assumed to lie in the x-y plane.) This film can be placed over a display to reduce the viewing angle range of the display, to give a private display mode. However, the film is not switchable so must it be physically positioned/removed to change between one viewing mode and another. Another drawback of the 3M microlouvre is that the there is never any privacy achieved in a plane parallel to the microlouvre grooves.
Louvred films may be manufactured by stacking many alternating sheets of transparent and opaque material and then cutting slices of the resulting block perpendicular to the layers. This method has been known for many years. For example, it is disclosed in U.S. Pat. Nos. 2,053,173, 2,689,387 and 3,031,351.
A process where a louvred film is cut continuously from a cylindrical billet of stacked layers is disclosed in U.S. Pat. No. RE27,617. U.S. Pat. No. 4,766,023 shows how the optical quality and mechanical robustness of the resulting film can be improved by coating with a UV-curable monomer and then exposing the film to UV radiation. U.S. Pat. No. 4,764,410 discloses a similar process where the UV-curable material is used to bond the louvre sheet to a covering film.
Other methods exist for making films with similar properties to the louvred film. For example, U.S. Pat. No. 5,147,716 discloses a light-control film which contains many elongate particles which are aligned in the direction perpendicular to the plane of the film. Light rays which make large angles to this direction are therefore strongly absorbed.
Another example of a light-control film is disclosed in U.S. Pat. No. 5,528,319. Embedded in the transparent body of the film are two or more layers parallel to the plane of the film, each layer having opaque and transparent sections. The opaque sections block the transmission of light through the film in certain directions while allowing the transmission of light in other directions.
The films described above may be placed either in front of a display panel or between a transmissive display panel and its backlight to restrict the range of angles from which the display can be viewed. In other words, they make a display ‘private’. However none of them can easily be switched off to allow viewing from a wide range of angles.
US 2002/0158967 shows how a light control film can be mounted on a display so that the light control film can be moved over the front of the display to give a private mode or mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantages that it contains moving parts which may fail or be damaged and which add bulk to the display.
Another known technique for switching between public and private mode with no moving parts is to mount a light control film behind a display panel and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect. Switchable privacy devices of such type are described in U.S. Pat. Nos. 5,831,698, 6,211,930 and 5,877,829. They share the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public or private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public than in private mode unless the backlight is made brighter to compensate.
A third known technique for providing a switchable public/private display is disclosed in U.S. Pat. No. 5,825,436. The light control device in this patent is similar in structure to the louvred film described earlier. However, each opaque element in the louvred film is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display is in its private mode; when the cells are transparent, the display is in its public mode.
A first disadvantage of this method is difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.
JP2003-233074 discloses a liquid crystal display which provides a conventional display of images from normal viewing angles. However, for relatively large viewing angles away from the normal to the display, a fixed image is displayed and this may be used to hide or “scramble” the normal image so as to provide a privacy mode of operation. The fixed image is achieved by having display regions with different alignment directions from each other, although all pixels operate in the same mode. A lower than normal drive voltage is used.
Although such an arrangement may be used to provide a privacy mode, it is not switchable. In particular, the relatively narrow range of viewing angles throughout which the non-fixed image can be seen cannot be changed if non-private operation is desired.
SID 05 Digest p1218 (Toshiba) discloses a display in which a liquid crystal layer for image display and an Ultra-Super Twisted (UST) liquid crystal layer are disposed between an entrance polariser and an exit polariser. The liquid crystal layers are illuminated by a backlight, and a switchable diffuser is provided between the backlight and the entrance polariser. The display may be switched between a wide display mode and a narrow display mode by switching both the UST liquid crystal layer and the switchable diffuser.
SID 92 Digest p401 (Toshiba) discloses an UST liquid crystal layer used to improve the viewing angle range of a TN liquid crystal display. However, the UST layer is not switched to alter the viewing angle range of the display.
GB 2 413 394 discloses a display having a display device and an additional liquid crystal display device that can be switched to control the viewing angle range of the display. The additional liquid crystal display device generally has an ECB (electrically controlled birefringence) liquid crystal layer, but in one embodiment the additional liquid crystal display device includes a cholesteric liquid crystal layer.